1. Field of the Invention
This invention relates to the coating of substrates, and in particular to a process for the efficient coating of substrates using chemical vapor reaction and gas plasma cleaning.
2. Description of the Related Art
The application of coatings onto substrates and other workpieces is required as a process step in many industrial fields. An example of such a process is the coating of a silicon wafer with a layer of Hexamethyldisalizane (HMDS). This coating process is used to promote the adhesion of organic layers such as photoresist to the inorganic silicon wafer. The HMDS molecule has the ability to adhere to the silicon wafer and also to be adhered to by an organic additional layer. For example, silicon wafers would be baked for 30 minutes in a 150 C. oven for 30 minutes to dehydrate them. The silicon wafers would then be sprayed with HMDS. The excess HMDS would then be spun off of the silicon wafer. A typical process of this type would result in a HMDS monolayer on the surface of the silicon wafer.
A problem encountered with the above mentioned process was that if the silicon wafer was not sufficiently dry and not sufficiently clean prior to the application of HMDS, then residual moisture would interfere with the reaction of the HMDS to the silicon wafer. This would result in variations in the HMDS layer reaction and then could lead to voids in the subsequently applied next layer. Another problem with a process of this type is that HMDS would rapidly deteriorate when exposed to air and moisture, and thus such a process required a large amount of HMDS to provide a small amount of reaction.
The coating of substrates for biotech, semiconductor, and other applications may require sufficiently clean and dehydrated substrates and insertion into the process chamber of one or more deposition chemicals which have been preheated and/or vaporized prior to insertion. Biotech applications may require silane deposition onto glass and/or other substrates as a bridge to organic molecules. Among the silanes used are amino silanes, epoxy silanes, and mercapto silanes. These silanes are used in the adhesion layer between glass substrates and oligonucleotides. Oligonucleotides are a short DNA monomer. Substrates are coated with a monolayer of silane as a bridge between the inorganic substrate and the organic oligonucleotide. A silane coated substrate with an oligonucleotide layer is now a standard tool used in biotech test regimens. One area where this oligonucleotide layer is used is in the formation of DNA microarrays. A uniform and consistent silane layer leads to a more uniform and consistent top surface of the oligonucleotide layer, which in turn leads to more useful test results.
When silanes are used as boundary layers, and in other applications, a consistent defect free layer becomes very important. In order to minimize defects, the substrates may need to be cleaned very thoroughly. Also, residual moisture may need to be removed from the substrate prior to the reaction of a silane with the surface. However, it may be very desirable to have embedded hydroxyl ions in the surface in some processes to provide an anchor for a reacted compound.
What is called for is a process which cleans a substrate, allows for rehydration of the substrate to restore an anchor layer to the substrate, and also sufficiently dehydrates the substrate to remove residual moisture. What is also called for is an apparatus which is able to plasma clean substrates in the chamber into which the vaporized chemicals will be delivered, and an apparatus which can clean itself after such production runs using plasma.
Substrates coated with such a process have reduced contamination, have more consistent monolayers with better bonds to the substrate.